Harmonic generator using equally spaced energy levels



April 1965 c. M. KELLINGTON HARMONIC GENERATOR USING EQUALLY SPACEDENERGY LEVELS Filed June 28, 1963 FIELD 0mmzw V INVENTOR, CHARLES M-KELLINGTON United States Patent 3,177,445 HONIC GENERATOR USING EQUALLYSPAQED ENERGY LEVELS Charles M. Kellington, Point Pleasant Beach, N .J.,assignor to the United States of America as represented by the Secretaryof the Army a Filed June 28, 1963, Ser. No. 292,21

3 Claims. (Cl. 331-94) (Granted under Title 35, US. Code (1952), see.266) The invention described herein may be manufactured and used by orfor the Government for governmental purposes, without the payment of anyroyalty thereon.

The present invention relates to a harmonic generator whereintheharmonic frequencies are generated by the interaction of aquantum-mechanical system and an electromagnetic field.

In the field of frequency generation of millimeter and submillimeterWavelengths, it has been the general practice to employ frequencydoublers or harmonic generators for increasing the frequency of theavailable energy. The most recent advances include the varactor and thecrystal diode. Although such devices have served the purpose,considerable difficulty has been experienced in obtaining asubstantially stable, noise free signal as a useful power output. Suchdevices drastically fall-off in efficiency with the increase infrequency.

The recent development of themaser has demonstrated that microwave.energy may be amplified efliciently, to a useful power level without theintroduction of noise and instabilities, by the stimulated emission ofradiation in solid state devices.

The operation of a solid state maser is dependent upon the fact that insome materials atoms and molecules exist in certain quantized energylevels or states. These states arise from the interaction of themagnetic moments of the electrons with internal or external fields. Theelectrons possess magnetic moments due to the spins of the electrons intheir orbits. A material having a plurality of available energy levelswill have, under ordinary conditions, more atoms at the-lower energystates than in the upper. To obtain maser action'this situation isreversed by absorption of energy at the correctfrequency i.e. the upperstate is made more populated; after which radiation is then emitted bythose atoms when they return to the lower or natural states.

The instant invention, utilizing this ability of a solid state crystalto absorb and emit energy, pumps a crystal at one frequency whilepermitting the crystal to emit at a frequency which is the harmonic ofthe pump frequency.

It is an object of the present invention to provide a quantum mechanicalharmonic generator.

A further object of the invention is to provide a harmonic generator theefiiciency of which will not falloif with an increase in the operatingfrequency.

Still another object of this invention is to provide a device which canproduce harmonic generation in the millimeter and submillimeter range.

Yet another object of the invention is to provide a harmonic generatorwhich is inherently stable and noise free. i

The exact nature of this invention as well as other objects andadvantages thereof will be readily apparent from consideration of thefollowing specification relating to q the annexed drawing in which:

3,177,445 Patented Apr. 6, 1965 FIGURE 3 is an illustration of oneembodiment of the invention.

Referring now to the drawings, there is shown in FIG- URE 1 a graph, theabscissa of which represents the energy possessed by atoms in aparamagnetic crystal, while the ordinate represents the value of anexternal D.-C. magnetic field applied to the crystal. Four curves,designated E E E and B are plotted on the graph. These curves representthe available energy levels for the atoms' of a four-level paramagneticcrystal. The exact curves shown in FIGURE 1 approximately represent theavailable spin energy levels for the Cr ion in the Al O Cr O (ruby)crystal While under the influence of a D.-C. magnetic field which isoriented at approximately an angle of 65 with respect to the C oroptical axis of the crystal. As shown in the graph of FIGURE 1, theenergy levels are two-fold degenerate in a zero magnetic field. However,these levels, when under the influence of an external magnetic field,lose their degeneracy with respect to the spin as represented by thecurves E1, E2, E3, and E4.

The spacing between these levels varies with the amount of the externalmagnetic field. However, for some particular values of the magneticfield the spacing between some of the levels will be equal to eachother. As shown in FIGURE 1, at the value H, for the magnetic field, thedistance from E to E is equal to the distance from E to E For theparticular crystal mentioned above, i.e. ruby, and for the conditionsoutlined, the application of substantially 3430 gauss will result in anequal spacing between levels E and E and levels E and E These levels,which are shown in FIGURE 2, may be used to produce harmonic generation.

Under ordinary conditions the distribution of the atoms in the energylevels is such that the lower energy levels are more densely populatedthan the higher levels. The densities may be altered by applyingelectromagnetic energy of the proper frequency to the crystal such thatabsorption will take place in the crystal. Once the upper levels arepopulated above normal, emission of radiation will occur. when theseatoms return to their normal or ordinary states.

It has been determined, subject to quantum selection rules, that atomsbeing pumped to various energy states absorb energy according to thefollowing relationship:

where h is Planks constant, v is the frequency of the absorbed energyand E is the increase in energy of the atom. The reverse process is alsotrue; that is, the atom will give ofi energy of the frequency v when theatom decreases in energy of the amount AE. This value, AB, is aquantized value as shown in FIGURE 2 thereby permitting only certainvalues of AE to exist for a particular condition.

If the spacings between energy levels are equal, harmonic generation maybe produced by pumping, with energy at a single frequency vcorresponding to the energy level spacing AE, the atoms in a multi-stepprocess and then extracting energy from the system while permitting theatoms to return to the initial energy state in a single step. Thisprinciple is illustrated in FIGURE 2 which shows an arrow E whichrepresents a rise in energy of an atom from state E to state E and anarrow E which represents a rise in the energy of an atom from state E tostate E Energy will be emitted when the atoms return to level E fromlevel E Since the frequency v of the emitted radiation is proportionalto the change in energy E and since the spacing of the energy levels areequal i.e. E =E then the frequency of the emitted radiation will beequal to twice the frequency of the pumping energy.

For the ruby crystal mentioned above and for the conditions outlined inconnection with this crystal, the spacings E and E 4 correspondsubstantially to a frequency of 9425 me. Emission will take place at thesecond harmonic or substantially 18,850 me. which is the sum of E and Eand corresponds to E FIGURE 3 which illustrates one embodiment of theinvention shows a pump using a waveguide 11 to feed energy to a cavity12. The cavity 12 is so designed as to be resonant at both the pumpfrequency and the particular harmonic desired. Located in the cavity 12is a paramagnetic crystal 13, the C-axis of which is oriented withrespect to a magnetic field produced by magnetic poles 14 and 15. Theoutput of cavity 12 is connected to a waveguide 16 which feeds a load17.

The magnetic field produced by poles 14 and will be adjusted to a valuecorresponding to the value H of FIGURE 1. Of course, any value of H maybe picked so long as the energy level spacings through which the atomsare to bepumped are equal.

The cavity or the crystal will operate at room temperature; however, ifthe temperature of the crystal is lowered to obtain a greater populationat the lower levels, the efficiency of the device will generallyincrease. The crystal will also be shielded from thermal noise whenrefrigerated. However, it has long been recognized that the emission ofradiation in paramagnetic materials gencrates a very small amount ofnoise at room temperature.

The operating range of such devices is limited only by the strength ofthe available magnetic field and the energy level characteristics of theavailable crystals. Of course, present crystals are capable of emittingradiation in the microwave and infrared regions. Obviously, the range offrequencies may be extended as desired by using more than three levelsthereby producing larger harmonies and higher frequencies.

Stability is inherent since the crystal will resonate only at thefrequency represented by the quantized energy levels.

It should be understood, of course, that the foregoing disclosurerelates to only a preferred embodiment of the invention and thatnumerous modifications or alterations may be made therein withoutdeparting from the spirit and scope of the invention as set forth in theappended claims.

What is claimed is:

1. A generator for producing second harmonic frequencies comprising aparamagnetic crystal, means applying a fixed direct current magneticfield to said crystal, said magnetic field providing first, second, andthird energy levels in said crystal, the separation between said firstand said second energy levels being equal to the separation between saidsecond and said third energy levels, means for applying to said crystalelectromagnetic energy of the frequency corresponding to each said equalseparations, and means extracting from said crystal energy of thefrequency corresponding to the sum of said equal separations.

2. A harmonic generator comprising a paramagnetic crystal, meansapplying a fixed direct current magnetic field to said crystal, saidmagnetic field providing a plurality of distinct and consecutive energylevel separations, said separations being equal to each other, means forapplying to said crystal electromagnetic energy of the frequencycorresponding to each said equal separations, and means extracting fromsaid crystal energy of the frequency corresponding to the sum of saidequal separations.

3. A harmonic generator comprising a cavity resonant at a firstfrequency and a harmonic of said first frequency, a paramagnetic crystallocated in said cavity, means applying a fixed direct current magneticfield to said crystal, said magnetic field providing a plurality ofdistinct and consecutive energy level separations equal in number tosaid harmonic, each of said energy level separations being equal andcorresponding to said first frequency, means for applying to said cavityelectromagnetic energy at said first frequency, and means for extractingfrom said cavity energy at said harmonic frequency.

References Cited by the Examiner UNITED STATES PATENTS 2,909,654 10/59Bloembergen 3304 3,001,142 9/61 Mims 330-4 OTHER REFERENCES Geusic:Physical Review, April 1960, pages 129-130.

Arams: Proceedings of the IRE, January 1960, page 108.

Advances in Quantum Electronics, edited by Singer, Columbia UniversityPress, 1961, pages 612-618 relied on (Fontana et al.).

Fontana et a1.: Proceedings of the IRE, April 1962, pages 469-470.

ROY LAKE, Primary Examiner.

1. A GENERATOR FOR PRODUCING SECOND HARMONIC FREQUENCIES COMPRISING APARAMAGNETIC CRYSTAL, MEANS APPLYING A FIXED DIRECT CURRENT MAGNETICFIELD TO SAID CRYSTAL, SAID MAGNETIC FIELD PROVIDING FIRST,SECOND, ANDTHIRD ENERGY LEVELS IN SAID CRYSTAL, THE SEPARATION BETWEEN SAID FIRSTAND SAID SECOND ENERGY LEVELS BEILNG EQUAL TO THE SEPARATION BETWEENSAID SECOND AND SAID THIRD ENERGY LEVELS, MEANS FOR APPLYING TO SAIDCRYSTAL ELECTROMAGNETIC ENERGY OF THE FREQUENCY CORRESPONDILNG TO EACJHSAID EQUAL SEPARATIONS, AND MEANS EXTRACTING FROM SAID CRYSTAL ENERGY OFTHE FREQUENCY CORRESPONDING TO THE SUM OF SAID EQUAL SEPARATIONS.